This invention generally relates to tunable High-Temperature Superconducting (HTS) filters and, more particularly, to such filters wherein the center frequency can be tuned within a broad frequency range without performance deterioration.
Until the late 1980s, the phenomenon of superconductivity found very little practical application due to the need to operate at temperatures in the range of liquid helium. In the late 1980s ceramic metal oxide compounds containing rare earth centers began to radically alter this situation. Prominent examples of such materials include YBCO (yttrium-barium-copper oxides, see WO88/05029 and EP-A-0281753), TBCCO (thallium-barium-calcium-copper oxides, see U.S. Pat. No. 4,962,083) and TPSCCO (thallium-lead-strontium-calcium-copper oxides, see U.S. Pat. No. 5,017,554). All of the above publications are incorporated by reference herein for all purposes as if fully set forth.
These compounds, referred to as HTS (high temperature superconductor) materials, were found to be superconductive at temperatures high enough to permit the use of liquid nitrogen as the coolant. Because liquid nitrogen at 77 K (xe2x88x92196xc2x0 C./xe2x88x92321xc2x0 F.) cools twenty times more effectively than liquid helium and is ten times less expensive, a wide variety of potential applications began to hold the promise of economic feasibility. For example, HTS materials have been used in applications ranging from diagnostic medical equipment to particle accelerators.
An essential component of many electronic devices, and particularly in the communications field, is the filter element. HTS filters are well known to have a wide variety of potential applications in telecommunication, instrumentation and military equipment. HTS band-pass filters have the advantage of extremely low in-band insertion loss, high off-band rejection and steep skirts. HTS band-reject filters have the advantage of extremely high in-band rejection, low off-band insertion loss, and steep skirts. The advantages of both types of filters are due to the extremely low loss in the HTS materials. Commonly owned U.S. Pat. No. 6,108,569 (incorporated by reference herein for all purposes as if fully set forth) describes HTS mini-filters which utilize self-resonant spiral resonators as the basic building block. These HTS mini-filters have very compact size and light weight, which greatly ease the cryogenic requirement and thus increase the ability to be used in many applications.
Certain applications require filters to have frequency tuning capability. There are three primary methods known in the art to achieve frequency tuning capability. The first method, described in D. E. Oates et al, IEEE Trans. Appl. Supercond. 7, 2338 (1997), involves the use of a ferrite material. The major problem with using ferrite materials is that the Q-value of ferrite materials at cryogenic temperatures is too low compared to HTS materials. In other words, introducing ferrite material into HTS filters deteriorates the performance.
The second method, described in G. Subramanyam et al, NASA Agency Report No. NASA/TM-1998-207490, involves the use of ferroelectric materials. Ferroelectric material tuning has the same problem of low Q-value as the ferrite material tuning and, in addition, has a bias circuit problem. In order to tune the filter, a bias circuit is needed to apply a voltage across the ferroelectric material, which may deteriorate the filter""s performance.
The third method, described in T. W. Crowe et al, Infrared Phys. And Tech. 40, 175 (1999), involves the use of a varactor as a variable capacitance attached to the filter""s resonator. The problems of this approach are similar to those of the ferroelectric tuning, i.e. low Q-value and bias circuit problems.
One object of this invention, consequently, is to provide a tunable HTS filter without performance degradation caused by Q-value deterioration related to the use of foreign materials and/or bias circuitry. Thus, in accordance with one aspect of the present invention, there is provided a tunable HTS filter comprising:
(a) an enclosure having a first inner surface, a second inner surface spaced apart from and opposite to said first inner surface, and at least one other inner surface connecting said first and second inner surfaces to form said enclosure, wherein at least said inner surfaces of said enclosure are constructed of a conductive material, and wherein said enclosure is fitted with an input connector and an output connector;
(b) an HTS filter circuit within said enclosure, said HTS filter circuit comprising a substrate having a front surface spaced apart from and opposite to said second inner surface, a back surface in grounding contact with said first inner surface, an HTS filter element on said front surface, said HTS filter element comprising one or more HTS resonators, an input transmission line coupling said HTS filter element to said input connector, and an output transmission line coupling said HTS filter element to said output connector;
(c) a plate within said enclosure, said plate having a front surface spaced a distance apart from and opposite to said HTS filter circuit, and a back surface opposite to said second inner surface, wherein said front surface is covered with an HTS film on at least the portion of said front surface opposite said one or more resonators of said HTS filter element;
(d) an actuator connected to said plate and to one or more of said first inner surface, said second inner surface and said HTS filter circuit, said actuator defining said distance at which said front surface of said plate is spaced apart from said front surface of said HTS filter element, provided that said actuator connection is non-conductive between said plate and said HTS filter circuit; and
(e) a tuning controller connected to said actuator to adjust said distance between said front surface of said plate and said HTS filter element of said HTS filter circuit.
The aforementioned plate interacts with the magnetic field of the resonators in the HTS filter circuit, changing the resonant frequency thereof as the distance between the plate and the HTS filter circuit changes. The movement of plate thus xe2x80x9ctunesxe2x80x9d the center frequency of the HTS filter.
During the tuning process, however, the inter-resonator coupling may change as well, which in turn can cause the filter""s bandwidth and the shape of the frequency response to change. These side effects may deteriorate the filter""s performance, and another object of the present invention is to provide an HTS filter element that can compensate for these side effects. Thus, in accordance with another aspect of the present invention, there is provided an HTS filter circuit that includes one or more compensating inter-resonator coupling circuits to compensate for these potential side effects. More specifically, there is provided an HTS filter circuit comprising:
(1) a substrate having a front side and a back side;
(2) at least two HTS resonators in intimate contact with said front side of said substrate;
(3) an input coupling circuit comprising a transmission line with a first end coupled to a first one of said at least two self-resonant spiral resonators, and a second end for coupling to an input connector;
(4) an output coupling circuit comprising a transmission line with a first end coupled to a second of said at least two self-resonant spiral resonators, and a second end for coupling to an output connector;
(5) an inter-resonator coupling circuit comprising an HTS transmission line at least in part disposed between an adjacent pair of said at least two HTS resonators, said transmission line coupling said adjacent pair of HTS resonators;
(6) a blank HTS film disposed on said back side of said substrate; and
(7) a film disposed on said blank HTS film as a grounding contact to an enclosure for said HTS filter circuit.